How to make preset for RGB curves in compositor
The properties of every point of RGB curves node can be accessed using the following snippet. Generally, the location of the point in XY plane is useful. Hence to access that property:

(RGB_Curves_Node).mapping.curves[0].points[id].location_x


Similarly,

(RGB_Curves_Node).mapping.curves[0].points[id].location_y


id is used to access which point you want the property from. Also, the 'R', 'G', 'B' and 'C' curves are separated by id 0, 1, 2, 3 respectively.

As you want to save the data as a preset, you would probably want to do something like

preset = [[], [], [], []]
for i in range(0, 5):
    for point in bpy.context.scene.node_tree.nodes['RGB'].mapping.curves[i].points:
       preset[i].append((point.location.x, point.location.y))